It was found in a previous work that the Al–Si alloy could spontaneously infiltrate into carbon preforms in air. In this study, the initiation stage of the infiltration process was investigated in detail through two different infiltration experiments. In one experiment, carbon preforms were fully dipped into an alloy bath that was exposed to air, and in the other experiment a carbon preform was only partially dipped into an alloy bath that was protected with a flowing Ar or N2 gas. Experimental results have suggested that the initiation of infiltration is controlled by the pressure of oxidizing gases such as O2 or CO at the infiltration front and is not affected by the presence or absence of N2 gas. The critical pressure of oxidizing gases is estimated to be on the order of 10−4 atm for systems investigated in our experiments. An effective way to reduce the O2 or CO pressure is to flush a preform with nonoxidizing gases during or before infiltration, or to use an active metal to reduce the O2 pressure and thus the corresponding CO pressure.